In order to address the physiololgical role of Tumor Necrosis Factor/ Lymphotoxin subfamily of cytokines we have generated, in collaboration with German and Russian laboratories, three distinct murine strains with deficiences in the TNF/LT locus. These include single deficiency in the LT-beta gene, double deficiency in LT-alpha/LT-beta and triple deficiency in LT-alpha/TNF/LT-beta (entire TNF/LT locus). All these mice have been generated using Cre-loxP recombination system starting from a single targeting construct. Multiple deficiencies in the TNF/LT locus cannot be obtained by crossing single knock-out mice, because the three TNF/LT genes are very tightly linked in the genome. All three strains of mice generated are immunodeficient and reveal major defects in lymphoid organs, including complete lack of Peyer's patches and lack of the most of lymph nodes, lack of marginal zone in spleen accompanied by poor T, B cell zone separation. They do not develop normal germinal centers upon immunization. Surprisingly, LTb-deficient mice revealed phenotypic defects in the development of the peripheral lymphoid organs similar but not identical to those reported earlier for the LTa- deficient mice. This suggested that the current scheme of ligand- receptor interactions, involving LTa and LTb, is incomplete. Other features include substantial lymphocytic infiltrates in parenchymal organs. Thymi appear normal even in triple deficient mice. We are currently using these models to identify genes and gene products responsible for developmental defects in spleen. AIDS TITLE: Developing New Models for Murine Models for AIDS Research- TNF/LT Triple Knock-out Mice.